The influence of crystal structures on the performance of CoMoO4 battery-type supercapacitor electrodes

CoMoO4 is a promising battery-type supercapacitor electrode material that can offer relatively high storage capacity and cycle stability. In this work, we investigate the role of the crystalline phase of CoMoO4 in determining these performance parameters. The hydrate phase of CoMoO4 was synthesized on a nickel foam substrate via hydrothermal reaction with subsequent annealing under an inert atmosphere leading to the formation of the β-phase CoMoO4. Similar nanoplate morphologies were observed in all of the samples. The hydrate-phase CoMoO4 demonstrates larger specific capacity than the annealed β-phase CoMoO4. Besides, the samples synthesized at lower temperatures have better rate capability than the sample annealed at higher temperatures. However, the hydrate phase had worse long-term stability compared to the β-phase samples.


Supplementary Figures
The interlayer spacing is calculated by Bragg's law (Equation S1) based on the data extracted from the XRD.The interlayer spacing of the (-1-11) (112) and (220) planes of the hydrate-phase CoMoO 4 is 0.426nm, 0.315nm and 0.301nm, respectively.The interlayer spacing of the (001) and (002) plane of the -phase CoMoO 4 is 0.590 nm and 0.331nm, respectively.The interlayer spacing matches with the reference, and confirms the more open structure of the hydrate phase [1][2][3].A small amount of -phase CoMoO 4 beyond the detection limit of XRD was detected.

𝛼
Table S1: Measured plane spacings from electron diffraction patterns in the hydrate phase for CoMoO 4 .Note that errors were calculated using the third significant digits in spacings but only two are reported herein to reflect accuracy.

Supplementary Calculation Details Interlayer Spacing
The interlayer spacing of different planes of the hydrate phase and β phase is calculated by Bragg's law: is the spacing between planes in the crystal lattice, λ is the wavelength of the X-ray and θ (˚) is the angle of incidence.

Cyclic Voltammetry Fitting
The peak current (i) and the scan rate (mV/s) can be fitted by the following equation [14,15]: a and b are adjustable parameters between 0.5 and 1.
The value of a and b depends on the charge storage mechanisms and reaction kinetics [16].If the value of b is close to 0.5, the process is assigned to a a diffusion-controlled process, and if the value of b is close to 1, the process is assigned to a a diffusion-free process.

Capacity
From the GCD curves, the mass-specific capacity of the electrode can be calculated by the following equation: Equation S3 =  ×  Q is the capacity (C/g), I (A/g) is the discharging current density, t (s) is the discharging time.

Electrochemical Surface Area (ECSA)
Quantifying the ECSA of an electrode helped in knowing the area of the reacting interface, therefore, normalizing current by ECSA was an important method to assess the intrinsic electrochemical activity of an electrode.The ECSA was calculated by the following equation: The C DL is the double-layer capacitance of the electrode.C S is the specific capacitance, and in our case, because the experiments were performed in an alkaline electrolyte, Cs was chosen as 0.04 mF/cm 2 [17].C DL of an electrode was estimated by CV scan in a small potential window in a non-Faradaic region [18].To gain accurate and comparable estimation of the C DL of the four samples, CV scan was first performed at 10mV/s from 0V to 0.6 V (vs.Hg/HgO) to make sure the samples were in the same condition before the ECSA estimation.Then, the CV scanning was performed at the same potential range for all CoMoO 4 samples.

Figure
Figure S1: X-ray diffraction patterns of the as-synthesized and 450˚C CoMoO 4 .

Figure S2 :
Figure S2: Low-magnification SEM images of the a) as-synthesized, b) 200 o C, c) 350 o C, and d) 450 o C samples.

Figure S4 :
Figure S4: Plots of i v -1/2 and v 1/2 of as-synthesized and 450C CoMoO 4 .The values of k 1 and k 2 can be obtained by plotting and fitting i/v 1/2 vs. v 1/2 .

Table S2 :
Measured plane spacings from electron diffraction patterns in the α phase for CoMoO 4 .Note that errors were calculated using the third significant digits in spacings but only two are reported herein to reflect accuracy.

Table S3 .
Fitted b values of redox reaction peaks of all the samples:

Table S4 :
Comparison between the hydrate-phase CoMoO 4 and similar materials.